An L5 Surprise

Ref:
http://www.gpsworld.com/gnss-system/gps-modernization/news/an-l5-surprise-10245?print=1

An L5 Surprise
July 20, 2010
By: Don Jewell

It has long been accepted that we may reasonably expect any new
technical device to have some growing pains. If you examine the history
of the space program you will discover evidence of this. In the case of
the first GPS IIF vehicle on orbit we do not have a 12,552-mile
screwdriver. but we do have dedicated software and systems engineers at
Boeing and in the U.S. Air Force that will solve the issues that crop up
and eventually present the world with a stable PNT platform.

I bring this to your attention because researchers at the German
Aerospace Center (DLR) say they have found a small variance in the L5
signal on IIF-1. The signal variation results in no more than a
5-centimeter error with a predictable periodicity of about six hours.
While observing the IIF, DLR also reports that the signal appears to be
“hot” or stronger than anticipated or advertised by about 1/2 db.

Initial reaction from the GPS Wing and Air Force experts at Schriever
AFB is that the signal fluctuation appears to be temperature-related, as
the periodicity correlates directly to the temperature extremes the
satellite is experiencing at this time of year in its MEO orbit. It is
being investigated as a matter of course during the standard checkout of
the satellite, which will continue for about another four weeks
according to the original checkout schedule.

The GPS Wing is confident that all the IIF signal specifications will be
met by the time the satellite is set healthy in about a month’s time and
they will be able to move forward with the IIF launch schedule as
planned. Obviously this could be perturbated by having to make
corrections or adjustments to the satellites still to be launched, but
this is normal procedure, and some leeway to correct anomalies is built
into the schedule for the first few launches of any new satellite system.

Privately, one official commented, "It turns out that no one has ever
made this measurement before. The Galileo SVs can only broadcast from 2
transmitters at a time so they are combining two E5 signals generated
from the same transmitter. Furthermore, their data was collected during
max Beta meaning when they weren't in eclipse (less thermal variance)."

The press release from the GPS Wing reads as follows.

SMC Update. July 19 – “Officials from the Air Force Space Command, Space
and Missile Systems Center’s Global Positioning Systems Wing announced
the “on-orbit checkout” of the first IIF satellite is progressing as
scheduled.
“Nearing the half way point of its 90-day checkout period, GPS IIF SV-1
also known as SVN 62/PRN 25, is currently broadcasting the same L1 and
L2 signals as previous GPS satellites and the new safety of life signal
known as L5. All three signals being broadcast from SVN 62 are set
unhealthy while experts monitor the quality and characteristics of the
signals and the performance of the satellite.

“During the initial phase of testing, [DLR] combined L1, L2, and L5
signals in a technique used to characterize a number of known and
modeled error sources from the signals. This three-frequency combining
technique helps isolate “other” sources of location error, such as
multi-path (when more than one path exists for signals to travel before
reception), receiver errors, satellite induced errors and unmodeled
phenomena. The L1, L2 and L5 signals from SVN-62 are operating nominally
but DLR noticed higher residual errors than expected compared to
previous somewhat similar measurements from Galileo’s GIOVE-A R&D satellite.

“The GPS Wing at Los Angeles Air Force Base has corroborated DLR’s
results and is investigating root cause to share a deeper understanding
of this new signal’s behavior with the user community. The causes of the
phase variation are still being investigated, but they are likely the
result of sensitivities to changes in the satellite’s thermal
environment. SVN 62/PRN 25 is currently experiencing periods of both
sunlight and total darkness (known as eclipse season) as the satellite
orbits the Earth and traverses through Earth’s shadow. Tests to
characterize the satellite’s performance during continuous sunlight
exposure will continue after the current eclipse season ends later this
month.

“Typical GPS receivers using stand-alone (single signal – L1, L2 or L5)
or combinations of L1, L2 and L5 signals as part of their navigation
solution will not be affected by this small phenomenon. The Air Force is
committed to maintaining excellence in GPS navigation and timing
services and to working with the user community to best use and exploit
the new modernized GPS signals.

"When tests of this new generation of GPS satellites have been completed
and Air Force leadership gives approval, the satellite signals will be
set healthy and will operate as specified in the Interface Control
Documents (ICDs).”